The invention relates to an oil recovery process in which oil is displaced by injecting a mixture of CO.sub.2 and aqueous liquid. More particularly, the invention relates to pre-forming such a mixture for coinjection in a manner significantly reducing its corrosivity, by including in the mixture an effective amount of dissolved monovalent cationic salt of carbonic acid.
In one aspect, the present invention provides an improved way of conducting the process of the type described in U.S. Pat. No. 2,875,833. That patent relates to an oil recovery process in which oil is displaced by injecting an aqueous solution which is substantially saturated with respect to carbon dioxide. In the present process, however, the saturated CO.sub.2 -solution can be mixed in any proportion with undissolved CO.sub.2 (e.g., it includes a so-called CO.sub.2 water alternate gas (WAG) process).
As stated in Enhanced Recovery Week (ERW), Sept. 9, 1985, "Carbonated waterflooding was largely dropped as an enhanced oil recovery (EOR) technique after initial investigations in the 1950s and Amoco's projects may be signaling a revival of interest (ERW, 4/29/85)". In the Nov. 25, 1985 ERW, it is indicated that Shell Western E & P plans a carbonated waterflood in the South Wasson Clear Fork Unit of the Wasson 72 field, making it the second company planning a carbonated waterflood. But, it is stated that, "Instead of injecting highly corrosive carbonated water, Shell will alternate small CO.sub.2 slugs with large water slugs, which will combine in the near wellbore reservoir into carbonated water."
The corrosivity of carbonated water is well known. U.S. Pat. No. 2,964,109 describes a utilization of carbonated water for acidizing a wellbore to remove a skin or layer formed during the drilling of the well. In that process carbon dioxide and water are injected into the well and held there under pressure until the pressure in the well begins to drop rapidly indicating a disintegration of the skin. Papers SPE 10685 and Canadian Institute of Mining (CIM) 83-34-17 discuss the dissolution of calcareous sandstones and carbonates by carbonated water.
The National Association of Corrosion Engineers Basic Corrosion Course, 1973, indicates that condenser corrosion is usually the result of dissolved carbon dioxide. It states that, "The CO.sub.2 is released from carbonates in the boiler and being volatile passes through the turbine into the condenser where it dissolves in the water, producing a low pH (acid conditions)." Such conditions cause thinning and grooving of the tubes if protective measures are not taken. And, the usual remedy is indicated to be making the solution alkaline to about pH 8.5 to 8.8 by additions of ammonia-type compounds such as morpholine or cyclohexylamine.